Mental retardation: 33% to 64% of children with ASDs also had mental retardation, less than the 70%-75% previously thought.

Gender difference: For every girl affected, three to seven boys were; the rate varied by state. It has been estimated that boys are four times as likely to have an ASD as girls.

Racial differences: Most sites reported similar rates in black and white children, though in the 2002 report, five of the 14 sites found that white children were more likely to have autism than black children. Data on Hispanic children showed lower rates.

The number of children who have an autism disorder — as many as 1 in every 150 — is significantly higher than previously thought, according to a new federal report being billed as the most complete assessment to date.

Earlier estimates placed the rate at 1 in 166 children. But Catherine Rice, lead author of the analysis released Thursday by the U.S. Centers for Disease Control and Prevention, says the previous estimates were based on smaller studies, some from other countries, using different study methods.

Advocates for people with autism say the results should help them push for more services. The findings may further stoke the impassioned debate over causes: Some blame thimerosal, a mercury-based preservative that was used in infant vaccines up until six years ago. Scientists concede that they don't know the cause but say the science doesn't support a link to vaccines.

For the new CDC report, researchers drew on information gathered in 2000 and 2002 by a multistate surveillance network on 8-year-olds who were identified as having an autism spectrum disorder (ASD). The category includes autism, PDD-NOS (pervasive developmental disorder not otherwise specified) and Asperger's disorder. All are marked by problems with language and communication and often are accompanied by repetitive or unusual behaviors. ASDs begin before age 3 but may not be diagnosed until later.

Researchers focused on 8-year-olds because most children with such disabilities will have been identified by that age, Rice says.

The 2000 study, which examined children who were born in 1992, involved sites in six states and 1,252 children with ASDs. The prevalence averaged 6.7 children out of every 1,000.

The 2002 study, which focused on children who were born in 1994, involved 2,685 with ASDs at sites in 14 states and found that autism prevalence ranged from a low of 3.3 in 1,000 children in Alabama to 10.6 in 1,000 in New Jersey. The overall average was 6.6 in 1,000 children, or about 1 in 150.

Estimates differ because ASDs vary and some states have better diagnosis and tracking, Rice says.

Peter Bell, president of Cure Autism Now, says the report is "confirmation of our worst fears, and that is that autism is on the rise. Every two or three years, we're given an estimate that is higher than the previous one."

CDC Director Julie Gerberding said in a statement that it's still unknown whether there's an actual increase in autism or just better studies. "We do know, however, that these disorders are affecting too many children," Gerberding said.

States facing rising autism rates have struggled to provide services. Rice says the new information can be used by communities to help plan for the future needs of these children. "We know the best outcomes come when kids are identified early," Rice says.

The revised estimates will "increase awareness and hopefully get legislation to follow that will give services to those with autism," says Marguerite Colston of the Autism Society of America.